The need for optical-quality single crystals of materials exhibiting non-linear optical properties is well known. Potassium titanyl phosphate (i.e., KTP) is particularly useful in nonlinear optical devices (see, e.g., U.S. Pat. No. 3,949,323). U.S. Pat. No. 3,949,323 discloses the preparation of crystals of compounds such as potassium titanyl phosphate by hydrothermal methods. Hydrothermal crystal growth processes have typically been high pressure processes involving crystal formation from a growth medium comprising mineralizer solution. A nutrient source such as a polycrystalline composition of the same material is provided, and seed crystals are often used to provide nucleation sites. Hydrothermal methods of crystal growth are considered particularly advantageous relative to flux methods of crystal growth for certain applications since they produce crystals having lower susceptibility to laser damage.
Hydrothermal methods typically require expensive reaction vessels to withstand the temperatures and/or pressures associated with crystal growth conditions and can involve long crystal growth periods to achieve crystals of the desired size and optical quality. Additionally, almost all hydrothermal crystals require growth in noble metal containers due to the corrosive nature of the growth solution, which also adds to the expense of the vessel. (Quartz is an exception, since it is commonly grown under hydrothermal conditions directly in steel autoclaves where corrosive attack of the autoclave is minimal because the common mineralizers react with iron to form sodium iron silicates which adhere to the autoclave wall and inhibit further corrosion.) See, Laudise et al., Journal of Crystal Growth 140 (1994) 51-56.
While substantial progress has been made in reducing the temperatures associated with hydrothermal crystal growth processes (see, e.g., U.S. Pat. No. 5,066,356), the relatively high reaction pressures typically associated with the hydrothermal process can still limit the practical application of the process. As is recognized in the art, vessels with noble metal liners have generally been limited to diameters of about 1 inch (2.54 cm) or less, and for larger diameters a sealed (often welded) noble metal can has been used to contain the corrosive growth solution. This generally requires balancing the pressure between the can and the autoclave with non-reactive fluid (e.g., water) at a fill to approximately equal the pressure at the operating conditions. (See Laudise et al., Journal of Crystal Growth 140 (1994) 51-56). With some fluid systems it is not possible to balance the two pressures during the entire heating and cooling cycle, and in those cases the gold can becomes deformed during the process. Deformation can result in container leaks, or problems in removing the container from the autoclave. When the precious metal configuration is used for crystal growth, the non-reproducible deformation results in non-uniform fluid circulation patterns and can effect the quality of the resulting crystals.
In addition to the effect that the deformation of welded cans may have on the reproducibility of the fluid circulation patterns during crystal growth, the deformation makes it difficult to insert thermocouples into the reaction chamber to measure the growth conditions accurately. Because of this difficulty, the general procedure used for the welded can technique is to use thermocouple wells in the heavy wall of the outer pressure containment vessel. As a consequence of the variations in location of the thermocouples and differences in external insulations, the actual growth temperatures and the temperatures differentials between sections of the reaction zones are not accurate and can lead to reproducibility problems.
In any case, hydrothermal crystal growth using corrosive fluids at high temperatures and pressures (typically about 500.degree. C. and 10,000 psi (68.9 kPa)), often use a noble metal container inside a base metal high pressure autoclave. In small diameter systems (e.g., 1 inch (2.54 cm) diameter or less) it has been possible to use a precious metal liner combined with precious metal gaskets and a clad head section. This sealing configuration is bolted down to the vessel using a variety of bolting arrangements. However, when it becomes desirable to scale up and increase the inner diameter of the vessel, the bolting forces used must be increased by a significant factor to balance the increased internal force on the head surface. The larger bolting forces combined with moderate temperatures over time can result in partial welding of the precious metal seal surfaces. As a result, the seal surfaces are damaged when the vessel head is removed from the vessel at the conclusion of the run. Platinum and its alloys are especially susceptible to pressure welding (see R. A. Laudise and J. W. Nielsen, J. Solid State Physics, 12 (1961) 161).
Thus, there is a need for vessels that avoid the problems associated with the sealed can reactor system, and the vessel size limitations of the conventional noble metal lined reactors. The need exists to increase the capacity of hydrothermal crystal growth vessels, and at the same time provide safe, reliable and economical seals for such larger systems.